Fix regressions in `Sync.clone`, fix reading hashes in `.git/HEAD`, change API to reset the store and start to improve 32bit support

• Allow some references to contain pointer to other references (#96)
• Improve the support for 32bit architectures (#97)
• Add Reference.pp_head_contents and Reference.equal_head_contents.
• Remove Store.clear and replace it by Memory.clear, Memory.clear_all
  and FS.remove. This let users have a finer control over the memory
  consumption of the program over time (related to #90)
• Rename all pp_hum functions into pp.
• Fix regression in Sync.fetch and add unit-tests (running only in slow mode).
• Fix reading of .git/HEAD when the contents is a commit hash.
• Depends on Stringext for all the extra string function needed.